1. Field of the Invention
The present invention relates to an article comprising a surface of light metal that has been provided with an alkali-resistant protective coat, to a method of producing it, and to its use.
2. Discussion of Background Information
Metallic aluminum and its alloys are a much-utilized material in industry, in usage and in the consumer sector. The exclusive reason for this widespread use is that, in spite of the non-precious nature of the metal, it has a relatively high resistance under ambient conditions on account of its capacity for surface passivation. In the course of this passivation an oxide layer is formed which is sufficiently impervious to withstand corrosive attack by moisture. If, however, conditions arise which attack the oxide layer, then the passivation is removed, and the corrosion rapidly advances. This applies to all acidic and basic attacks, i.e., at a pH above or below pH 7. The reason is the amphoteric nature of aluminum oxide, which results in its solubility both in acids and in bases.
Any chemical attack on a solid body is dependent for its reaction rate on the size of the surface. Where there is a high specific surface area, the rate of dissolution is rapid.
A long-established method of protecting aluminum surfaces, especially in conjunction with decorative effects, is that of anodic oxidation. One of the most common methods, known under the brand name “Eloxal”, is widespread in industry and practice. In that method, an electrical potential is used to produce—with relative rapidity—an oxide layer into which, as and when required, it is possible to incorporate pigments and produce color effects. The structure of this oxide layer, however, is not compact, but instead more porous, and so offers a high surface area to—for example, an alkaline attack. Consequently these anodic layers are very sensitive to bases. Therefore, at the kind of pH levels prevailing in a machine dishwasher, for example, they are dissolved or attacked at a more or less rapid rate, or they fade.
Because in particular the surface effects produced via anodic oxidation are very important for decorative effects, the production of dishwasher-resistant decorative coats on aluminum surfaces, for example, of the kind that are needed for household utensils, for example, is of great importance.
The literature describes sol-gel processes in which hydrolytically stable oxides are applied in liquid form to a surface and then stabilized thermally, by compaction or hardening, for example.
The attempt to employ such processes in order to stabilize anodic oxide layers, however, has proven not to be practicable, since it is not possible to close off fully the porosity which is inherent in the anodic layers. Consequently it is not possible to prevent penetration of an alkaline medium into the layer and to the interface with the aluminum. Furthermore, it has been found that, when such systems are heated for stabilization to temperatures above 200° C., contraction effects arise in the porous layer that result in cracking. This reduces the chemical resistance still further. The oxides used for these systems have been titanium dioxide or zirconium oxide, both oxides known for their high alkaline resistance.
The object of the present invention, therefore, was to develop a coating system and a technology which on the one hand allows anodically oxidized surfaces to be stabilized but on the other hand is suitable for application to aluminum surfaces even without prior anodic oxidation. Furthermore, the coat ought to be able to be applied in a liquid form and stabilized by a suitable method. The protective coats obtained are to be crack-free and alkali-resistant. A further objective is the production of a transparent protective coat.
This object has been achieved by an alkali-resistant protective coat which is applied to the surface of the light metal and comprises a) an oxide layer of silicon and boron as basecoat and b) an oxide layer of silicon as vitreous topcoat. In this way it is possible, on light metal surfaces, even on porous surface layers of the light metals, to obtain surprisingly crack-free and alkali-resistant protective coats, since the perviousness to alkaline media is strongly restricted, and so the protective coat offers excellent protection against attack by alkaline media. Since the double coat can be applied by wet-chemical methods, the production of the protective coat is simple and inexpensive, moreover. Transparent protective coats can be obtained.